1. Field of the Invention
This invention relates to a zoom lens, and particularly to a zoom lens suitable for a television camera, a phototaking camera, or a video camera, in which an aspherical surface is appropriately used in a portion of a lens system, whereby the zoom lens has good optical performance over an entire variable power range and has as large an aperture as the F-number of the order of 1.7 at the wide angle end and moreover a wide angle (a wide angle end angle of view 2.omega.=58.degree.-70.degree.) and as high a variable power ratio as a variable power ratio of the order of 12-35.
2. Related Background Art
Zoom lenses having a large aperture and high variable power and moreover having high optical performance have heretofore been required of television cameras, phototaking cameras and video cameras.
In addition, particularly in color television cameras for broadcasting, importance has been attached to operability and mobility and in compliance with these requirements, a compact CCD (solid state image pickup element) of 2/3 inch or 1/2 inch has become the mainstream as an image pickup device.
This CCD has its entire image pickup range having substantially uniform resolving power and therefore, for a zoom lens using it, it is required that the resolving power be substantially uniform from the center of the image field to the periphery of the image field.
For example, it is required that various aberrations such as astigmatism, distortion and chromatic difference of magnification be corrected well and the entire image field have high optical performance. Further, it is desired that the zoom lens have a large aperture, a wide angle and a high variable power ratio and moreover be compact and light in weight, and have a long back focus because a color resolving optical system and various kinds of filters are disposed forwardly of image pickup means.
Among zoom lenses, a so-called four-unit zoom lens comprising, in succession from the object side, four lens units, i.e., a first lens unit of positive refractive power for focusing, a second lens unit of negative refractive power for focal length change, a third lens unit of positive or negative refractive power for correcting the image place changing with a focal length change, and a fourth lens unit of positive refractive power for imaging can be relatively easily made to have a high variable power ratio and a large aperture and therefore is often used as a zoom lens for a color television camera for broadcasting.
Of four-unit zoom lenses, a four-unit zoom lens having a great aperture ratio and high variable power in which F-number is of the order of 1.6-1.9 and the variable power ratio is of the order of 13 is proposed, for example, in Japanese Laid-Open Patent Application No. 54-127322.
In a zoom lens, to obtain a great aperture ratio (F-number 1.7-1.8) and a high variable power ratio (variable power ratio 12-35) and a wide angle (a wide angle end angle of view 2.omega.=58.degree.-70.degree.) and moreover, high optical performance over the entire variable power range, it is necessary to set the refractive power and lens construction of each lens unit appropriately.
Generally, to have a small aberration fluctuation over the entire variable power range and obtain high optical performance, it becomes necessary to increase, for example, the number of lenses in each lens unit to thereby increase the degree of freedom of design in aberration correction.
Therefore, if an attempt is made to achieve a zoom lens having a great aperture ratio, a wide angle and a high variable power ratio, there unavoidably arises the problem that the number of lenses is increased and the entire lens system becomes bulky, and the desire for compactness and lighter weigh cannot be met.
Also, regarding the imaging performance, firstly, the changing of the point at the center of the image field whereat the image contrast is best, i.e., the so-called best image plane, resulting from a focal length change poses a problem. This is attributable chiefly to the changing of spherical aberration resulting from a focal length change. This spherical aberration influences by the cube of the aperture in the area of third-order aberration coefficient and therefore, is the greatest problem against a larger aperture.
Generally, the changing of spherical aberration resulting from a focal length change, when the zoom ratio is Z and the focal length of the wide angle end is fw, tends to become under (minus) relative to the Gaussian image plane from the wide angle end at which spherical aberration is 0 to the vicinity of a zoom position fm=fw.times.Z.sup.1/4, as shown in FIG. 33 of the accompanying drawings. When the vicinity of the zoom position fm=fw.times.Z.sup.1/4 is passed, the under amount becomes small and at a certain zoom position, it becomes 0 and now tends to become over (plus).
In the foregoing, fw is the focal length of the wide angle end, and Z is a zoom ratio.
The changing of spherical aberration becomes most over (plus) near a zoom position fd=(Fno. w/Fno. t).times.ft at which F drop in which F-number becomes great (the lens system becomes dark) begins, and when this zoom position is passed, the over amount becomes small to the telephoto end, and becomes nearly 0 at the telephoto end.
In the foregoing, Fno. w and Fno. t are F-numbers at the wide angle end and the telephoto end, respectively, and ft is the focal length of the telephoto end.
As described above, particularly in a zoom lens having a position at which F drop begins, the control of spherical aberration on the telephoto side becomes very difficult.
Next, regarding the wider angle of a zoom lens, of the imaging performance, distortion becomes the greatest problem. This is because in the area of third-order aberration coefficient, distortion influences by the cube of the angle of view.
As shown in FIG. 34 of the accompanying drawings, distortion is considerably greatly under (minus) at the wide angle end (focal length fw). From the wide angle end fw toward the telephoto end (focal length ft), distortion sequentially becomes greater in the over (plus) direction, and passes a zoom position at which distortion is 0, and the over value becomes greatest near the zoom position fm=fw.times.Z.sup.1/4. From the focal length fm to the telephoto end ft, the over amount sequentially becomes smaller. This tendency becomes greater as the angle of view at the wide angle end becomes greater and therefore, when the wider angle of a zoom lens is contrived, the control of distortion on the wide angle end becomes very difficult.
In order to correct such changing of various aberrations well over the entire variable power range, the number of lenses in the lens unit for focusing or the focal length changing system has been increased to thereby correct it. This has led to the problem that the entire lens system becomes bulky and complicated.
Also, the introduction of an aspherical surface for the solution of such a problem is proposed, for example, in Japanese Laid-Open Patent Application No. 7-35978.
However, the specification of zoom lenses has been improved, and in a zoom lens having a great aperture ratio and moreover having a high variable power ratio beginning from a super-wide angle, the revision of the method of introducing an aspherical surface has become necessary.
In a zoom lens having a great aperture ratio and moreover having a high variable power ratio beginning from a super-wide angle, spherical aberration changes greatly on the telephoto side and distortion changes greatly on the wide angle side. To correct both of these aberrations well, it is necessary to apply an aspherical surface on to an appropriate lens surface in a focal length changing portion.